1. Field of the Invention
This invention relates to fiber optical sensors, and more specifically to evanescent-wave fiber optical sensors.
2. Description of the Related Art
Fiber optical sensors are analytical devices in which an optical fiber serves as a transduction device. With the advent of optical transducers, better electronics, and improved immobilization methods, fiber optical sensors are being increasingly applied to industrial process and environmental monitoring, food processing, and clinical applications. These fiber optical sensors have specific advantages such as geometrical versatility, remote-sensing capability, small dimensions, and light weight.
Evanescent-wave fiber optical sensor are a class of fluorescence-based fiber optical sensors that selectively measure the fluorescence of fluorescent materials immobilized onto a surface of the optical fiber by use of the evanescent wave. The evanescent wave is associated with an electromagnetic field propagating in the optical fiber and typically penetrates from a few tens nanometers to several hundred nanometers into the medium surrounding the optical fiber, when the cladding layer of the optical fiber is removed. This evanescent wave can locally excite the fluorescent materials when they are bound by molecules on or very close to the optical fiber surface. This fluorescence technique can lead to an efficient and selective immunoassay or hybridization assay.
However, evanescent-wave fiber optical sensors have demonstrated inadequate sensitivity. Specifically, this is attributed at least in part to the effects of a weak evanescent wave that only allows a small percent of its light to excite the fluorescent materials on or near the fiber surface.